Strained relations between the Conservatives and Liberal Democrats reached a new low yesterday in an increasingly bitter campaign ahead of next month's referendum on the voting system.

Doubts were expressed about whether the coalition would last after Liberal Democrat leader Nick Clegg launched his strongest attack on the Tory-led 'No' campaign and did not exempt Prime Minister David Cameron from criticism.

Officially, both parties insisted it would be "business as usual" after the May 5 referendum. But insiders believe the relationship will never be the same again.

The Liberal Democrats raised the stakes yesterday as they sought to combat what Mr Clegg called the "lies, misinformation and deceit" of the 'No' camp. He said that opponents of the alternative vote (AV), including Mr Cameron, were the "death rattle of a right-wing clique who want to keep things the way they are".

Liberal Democrat deputy leader Simon Hughes intends to complain to the Electoral Commission about "untruths" from the 'No' campaign. And Liberal Democrat Energy and Climate Change Secretary Chris Huhne suggested his party might sue the 'No to AV' organisation unless it retracted its claims -- including that it would mean spending £135m (€154m) on electronic vote-counting machines.

Credibility

He warned that the credibility of Mr Cameron, Chancellor George Osborne and Foreign Secretary William Hague would be undermined unless the allegations were withdrawn.

"Let's be clear -- if they aren't prepared to come out with any substantiation for this extraordinary allegation that we're going to need voting machines for the system when none of the other countries actually have (them), there is a very simple legal redress.

"My colleague Simon Hughes is talking about getting the Electoral Commission to look at this, and there will be other legal means as well -- so they'd better come clean pretty fast."

Mr Huhne refused to say whether he might resign from the cabinet over the AV row.

Senior Tories and Liberal Democrats expected the two parties to diverge ahead of May 5, when they will do battle in elections to English councils, the Scottish Parliament and Welsh Assembly as well as on opposite sides of the AV campaign. But the acrimony of the battle has surprised both sides.

In some ways, it suited both parties to have a degree of what marketing men call "product differentiation".

If anything, the coalition had worked so well that there were fears in both parties that Mr Cameron and Mr Clegg were a bit too close for their parties' own good.